Grass Carp represent a significant threat to many freshwater ecosystems where they compete with native species for food, they can significantly alter habitats through their foraging behavior, and they can reduce catches of commercial fish and damage commercial fishing gear. The Sandusky River, a tributary to western Lake Erie in Ohio, has a relatively high abundance of Grass Carp compared to other Great Lakes tributaries. Federal, State, and University biologists, researchers, and managers invest significant resources each year in controlling Grass Carp in the Sandusky River to prevent the spread and establishment of this species. Environmental DNA (eDNA) has been successfully used to monitor aquatic invasive species invasive species
An invasive species is any plant or animal that has spread or been introduced into a new area where they are, or could, cause harm to the environment, economy, or human, animal, or plant health. Their unwelcome presence can destroy ecosystems and cost millions of dollars.

Learn more about invasive species , including invasive carp, in many freshwater ecosystems and there was interest in exploring the use of eDNA as an additional monitoring tool for Grass Carp in the Great Lakes. The U. S. Fish and Wildlife Service’s (USFWS) Whitney Genetics Lab recently developed new quantitative PCR (qPCR) assays for Grass Carp and validated these assays in multiple laboratories. In this study, we conducted a thorough field evaluation of these assays in the Sandusky River with the hope of developing an eDNA occurrence model. This model could then inform sampling plans and future Grass Carp eDNA monitoring efforts. We collected water samples at three sites in the Sandusky River a total of six times in 2019 and 2021. At two of these sites (Wightmans Grove and Brady’s Island), we attempted to sample three distinct habitat types: main channel, side channel, and backwater. The third site near Tiffin, OH, had a single habitat type (main channel) and served as a control site where we did not expect to detect Grass Carp eDNA. We were able to detect Grass Carp eDNA during each season and in and in all habitat types. Overall detection rates were low, with an approximately 1.5% positive detection rate out of over 3,000 samples processed. The number of Grass Carp eDNA detections was roughly equal between Wightmans Grove and Brady’s Island. Due to the lack of positive detections, eDNA occurrence models did not yield anticipated results. Models showed that the probability of Grass Carp eDNA presence (ψ) was highest in Summer and Fall sampling events. The probability of collecting eDNA when present (θ) was high and did not differ among sites or habitat types. The probability of molecular detection (ρ) with qPCR was essentially 0 across all sites and habitat types. Although Grass Carp were present in the Sandusky River during eDNA sampling events (based on telemetry data), the actual number of fish in the system was unknown and may have been too low for adequate eDNA detections necessary to facilitate model development. Sampling events may have also missed biological events (e.g., spawning) where detections could have been higher. This study demonstrated that these markers can be used to detect Grass Carp in the field; however, we do not have enough data to determine the optimal sampling scheme for a long-term eDNA monitoring program for Grass Carp. Future efforts may need to focus on areas where Grass Carp are more abundant in order to achieve higher detection probabilities. In the future, we hope to explore alternate eDNA collection strategies to determine if we can achieve higher detection probabilities.